This application relates to ultraporous thin film membranes.
More particularly, this application relates to ultraporous thin film membranes formed of cross linked polymers, and to methods and techniques for the manufacture thereof.
Ultrafilters and ultraporous membranes are widely used for a wide diversity of filtration and separatory processes, as well as diffusion processes, such as dialysis and the like. It has long been known that the performance of such membranes is generally limited by the effective thickness of the membrane, since for a given type and pore size, the thicker the membrane (or the thicker the active layer of the membrane) the lower the permeability at the working pressure differentials in use. It has long been the objective of the art to enhance the performance of ultraporous membranes by reducing the thickness of the membranes or reducing the thickness of the active layer of the membranes.
The state of the art has been limited in its efforts to make thinner membranes and membrane active layers by the methodology by which such membranes are formed. Both integral and composite ultraporous membranes are known, and employed for a variety of uses. While numerous attempts have been made to make still thinner ultraporous membranes and thinner ultraporous active layers, the result has generally been on the one hand a loss of quality or filtration efficiency that precludes the effective usage of such membranes, or on the other hand the use of membranes which are exceedingly expensive and difficult to make.
Concomitantly, there has been considerable effort in the reverse osmosis, or hyperfiltration, membrane art on the development of thin film membranes for reverse osmosis and other high pressure procedures. In the development of these efforts, a number of thin films have been attained. Some are cross linked polymers, so that there is a substantial body of knowledge available to those of ordinary skill in the art.
It has been thought that it would be desirable to obtain ultraporous thin film membranes. See for example, Wrasidlo, U.S. Pat. No. 4,005,012. There is, however, no effective technique for the production of ultraporous thin film membranes available to use in the art, and no effective enabling disclosure of how such membranes may be made which has effectively placed such ultraporous thin film membranes in the hands of those of ordinary skill in the art or into the possession of the public.
The earliest reference to thin film reverse osmosis membranes appears to be the teachings of Cadotte, U.S. Pat. No. 3,926,798, based on the in situ polymerization of furfural alcohol on a porous support. A highly cross linked thin film membrane suitable for reverse osmosis results. Cadotte does not teach or contemplate controlling or limiting the cross link density, so that there is no possibility that an ultraporous condition can result.
Interfacial condensation polymerization has since been employed to obtain thin films with a variety of polymer systems. See for example, Morgan, Condensation polymers: By Interfacial and Solution Methods, Interscience, New York, 1965.
Among the polymer systems explored are the following: Polyethylene imine solutions cross linked with toluene diisocyanate (TDI), isophthaloyl chloride (IPC), or trimesoyl chloride (TMC); m-Phenylene diamine solutions cross linked with TDI, IPC, or TMC; cross linked aromatic polyamides, and such as the cross linked polyether condensation polymer of 1,3,5,-trishydroxyethyl isocyanate and furfuryl alcohol. None of these have been reported in ultraporous thin films, and the art has not recognized either the possibility of doing so or any methodology by which such an objective might be attained.
What the art has recognized is that all these polymers are subject to severe chemical attack under the usual conditions of use in reverse osmosis, and as a consequence of these factors, such thin film polymers have met with limited use.
It is seen that the thin film membranes of the prior art are semi-permeable reverse osmosis membranes, and that thin film membranes, i.e., down to mono-molecular thickness, are not known in ultraporous or other related uses which require that an extremely thin film be ultraporous in character.